It was revealed in recent years that highly unsaturated fatty acids such as eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA) have pharmacological effects, and they are hence used as raw materials for pharmaceutical products and health foods. These highly unsaturated fatty acids have a multiple number of double bonds, whereby being not easily obtainable by chemical synthesis. For this reason, most of the highly unsaturated fatty acids used for industrial applications are produced by extracting or purifying raw materials derived from marine organisms rich in highly unsaturated fatty acids such as fish oils. However, since an organism-derived raw material is a mixture of diverse fatty acids with the different numbers of carbon atoms, the different numbers and positions of double bonds and also component ratios of diastereomers, a content of highly unsaturated fatty acid is not necessarily high. Under such a circumstance, the selective purification of an intended highly unsaturated fatty acid from an organism-derived raw material has been always demanded.
Patent Literature 1 describes a method for purifying a highly unsaturated fatty acid or the alkyl ester thereof by, when treating a raw material containing highly unsaturated fatty acids or alkyl esters thereof by the thin film distillation method, supercritical gas extraction method and urea addition method, carrying out the supercritical gas extraction method later than the thin film distillation method.
Patent Literature 2 describes a method for purifying high purity eicosapentaenoic acid or the lower alcohol ester thereof wherein the method comprises subjecting a raw material containing highly unsaturated fatty acids such as EPA to vacuum rectification and mixing the obtained fraction containing EPA and a lower alcohol ester thereof with an aqueous solution of silver nitrate. The conditions for vacuum rectification are described as a pressure of 5 mmHg (665 Pa) or less, preferably 1 mmHg (133 Pa) or less, and a temperature of 215° C. or less, preferably 210° C. or less.
Further, Patent Literature 3 describes a method for producing eicosapentaenoic acid or the ester thereof in a concentration of 80% or more wherein the method comprises distilling stepwise a raw material containing highly unsaturated fatty acids or the alkyl ester thereof using a 3 or more-stage distillation column. The distillation conditions are described as 10 Torr (1330 Pa) or less, preferably 0.1 Torr (13.3 Pa) or less, and 210° C. or less, preferably 195° C. or less.
However, highly unsaturated fatty acids in a much higher concentration and purity than those obtained by the above conventional methods are demanded as raw materials for pharmaceutical products and health foods.
Highly unsaturated fatty acids exist in a form of cis or trans isomer. Most of the highly unsaturated fatty acids in vivo are the cis isomer but are sometimes converted from the cis isomer to the trans isomer due to heating or the like, during the purification process using an organism-derived raw material (Non Patent Literature 1). Consequently, highly unsaturated fatty acids industrially purified from organism-derived raw materials contain a certain amount of the trans isomer. However, it is reported that trans fatty acid causes health risks, particularly elevates the LDL cholesterol value, and increases the risk of cardiovascular diseases. In the US and Canada, it is obliged to label food products a trans fatty acid content.
Thus, it is demanded to develop, as a raw material for pharmaceutical products and health foods, a composition comprising highly unsaturated fatty acids, which comprises not only an intended highly unsaturated fatty acid in a high concentration but also a trans fatty acid in a reduced content as possible. However, the purification of highly unsaturated fatty acids has never been practiced so far with an attention to the diastereomer ratio.